The present invention relates to a measurement method with three-dimensional images using nuclear magnetic resonance (hereinbelow abbreviated to NMR), and in particular to an inspection method suitable for measuring simultaneously three-dimensional images of a plurality of regions.
A magnetic resonance imaging apparatus is one for obtaining tomographic images using nuclear magnetic resonance phenomena. Usually measurement is effected with two-dimensional images of a specified slice. However, in case where images of a large area are required, two-dimensional multi-slice images or three-dimensional images are used for measurement. In a measurement with two-dimensional multi-slice images magnetic resonance (NMR) signals from a plurality of slices are measured during a waiting time for magnetization recovery and a plurality of two-dimensional images can be obtained in a measurement time for one two-dimensional image. By this method, since a region constituting one image is in accordance with a region where spin should be excited, S/N decreases which decreases slice thickness. In addition, since it is feared that substantial excitation regions overlap on each other, no measurement can be effected in a gapless manner.
On the contrary, for three-dimensional images, a large region is excited and a plurality of tomographic images can be obtained by phase encoding. For this reason, signal measurement efficiency is high, S/N doesn't decrease remarkably, even if slices are thin, and measurement can be effected in a gapless manner.
The principle of the magnetic resonance imaging explained above is described in detail in "MRI diagnostic (basis and clinic)", Asakura bookseller's, pp. 69-78 (1988). A drawback of the three-dimensional measurement is that measurement time is long, because the number of repetitions increases.
However it is possible to shorten the measurement time by applying an ultra fast imaging method such as the echo-planar method, etc., described in Journal of Magnetic Resonance, 29, pp. 355-373 (1978), to the three-dimensional image measurement.
FIG. 2 is a pulse sequence diagram in case where the conventional echo-planar method is applied to the three-dimensional image measurement. In FIG. 2 the abscissa represents the time and the ordinate represents the intensity of RF pulses, gradient magnetic fields, etc. Further reference numeral 1 is an excitation RF pulse; 2 is a slicing gradient magnetic field applied in a first direction; 3 is a phase encoding gradient magnetic field applied in the first direction; 4 is a phase encoding gradient magnetic field applied in a second direction; 5 is a readout gradient magnetic field applied in the third direction; and 6 is a nuclear magnetic resonance signal.
The excitation RF pulse 1 is applied to an object to be inspected at the same time as the slicing gradient magnetic field 2 to excite a specified region. Spatial information in the first direction is given to the nuclear magnetic resonance signal 6 by applying subsequently thereto the phase encoding gradient magnetic field 3 in the first direction.
Then the nuclear magnetic resonance signal 6 is read out by applying thereto the readout gradient magnetic field 5 at the same time as the phase encoding gradient magnetic field 4. The nuclear magnetic resonance signal 6 consists of a plurality of echo signals, each of which has a peak, when the integral of the readout gradient magnetic field 5 is zero.
At this time, since each of the echos includes spatial information in the direction, in which the readout gradient magnetic field 5 is applied, and further applied magnitude of the phase encoding gradient magnetic field 4 differs for different echos, a plurality of echos including different spatial information for the application direction of the phase encoding gradient magnetic field 4 are measured.
The above procedure is repeated while varying applied magnitude of the phase encoding gradient magnetic field 3 and a three-dimensional image is obtained by subjecting the measured nuclear magnetic resonance signal to three-dimensional Fourier transformation.
In the ultra fast imaging method, spectroscopic imaging, etc., inhomogeneity of a static magnetic field below about several ppm, which is so small that it gives rise to no problem for usual imaging, lowers significantly S/N or spectroscopic resolution. Therefore it is desirable to effect a processing to improve homogeneity of the static magnetic field prior to these imagings. However, since static magnetic field distribution is distorted by characteristics of the magnet itself, influences of magnetic substances in the neighborhood, susceptibility distribution of an object to be examined itself, etc., the processing is generally not easy.
Usually a multi-channel coil system called shim coil is incorporated in a magnet for generating the static magnetic field in order to correct this inhomogeneity of the static magnetic field. The homogeneity of the static magnetic field in a region to be imaged is improved by superposing shim magnetic fields having various characteristics generated by this multi-channel coil system on the static magnetic field generated by the static magnetic field coil.
However, since shim coils can generate at most only magnetic fields of about third order, it is not possible to correct completely higher order distortion in the magnetic field due to the shape of a living body all, etc. over the whole magnet.
Therefore, heretofore, as described e.g. in Magnetic Resonance in Medicine, 18, pp. 335-347 (1991), regulation of the static magnetic field is effected so that distribution of the static magnetic field is uniform only in a region used for the imaging.
For example, when two slices distant by 8 cm from each other are imaged by multi-slice imaging, as indicated in FIG. 3, a set of shim currents are determined so that the homogeneity of the static magnetic field all over the region including the two slices is best and shim magnetic fields are generated according to these shim currents prior to the multi-slice imaging (Step 100). Thereafter different slices are imaged without varying the distribution of the magnetic field (Steps 200 and 300).